We propose a new turbine design using the pressure principle, forcing high pressure gas through long and shallow spirals of many turns. The gas performs work on long spirals of moderate width and very shallow depth. A gradual and close to linear drop of pressure occurs along the length of the spirals when the spirals are turning at a moderate angular velocity. Adiabatic expansion of the gas occurs gradually as the gas turns in the spiral, making the process more or less isentropic.
The slow expansion is unlike the rapid expansion of impact or reaction turbines, which instantaneously convert all pressure energy into kinetic energy as the gas expands through a diverging nozzle. This rapid expansion converts most of the pressure and heat energy of the gas into high speed gas flow, which can sometimes reach supersonic speed. Momentum of the gas flow is imparted to the turbine through impact on turbine blades. The impact makes the blade rotate around an axle. Many such blades together cause the turbine rotor to turn at a high speed. The disadvantage of the reaction or impact turbine is that the kinetic energy of the high speed gas is often reconverted back into heat, causing entropic losses to the heat engine. Our disclosure avoids entropic inefficiency by having the gas retain its pressure, as the gas works along a long spiral of moderate width and very shallow depth.
The advantages of our turbine are many. First, we have improved conversion efficiency of heat into kinetic energy. Second, the turbine is scalable in power by widening the spiral or by increasing the number of spirals in a stack of disks. Multiple flat disks can be bolted together to increase power capacity of the turbine. Third, we have simplicity of manufacturing. The spirals are mechanically cut by modern high precision techniques such as wire Electrical Discharged Machining (wire EDM). Fourth, the rotational speed of the turbine is much reduced, in the order of a thousand revolutions per minute (rpm), instead of the typical tens of thousands of rpm for a modern turbine.
This lower speed allows the turbine axle to be directly coupled to an induction or synchronous generator without the use of gears to reduce the rpm. The reduced rpm allows the turbine to be synchronized with 60 Hertz alternating current for easy phase tying of the electrical output of a synchronous poly-phase generator to the electric power grid. We shall describe how the turbine can be directly coupled into a synchronous poly-phase permanent magnet generator. Alternatively, we may use a poly-phase induction generator.
This small, simple, and economical turbine is suitable for power conversion using concentrated solar power as the heat source. Highly focused sunlight produces high temperature and pressure steam to drive the turbine. We describe also how we may use a fossil fuel such as natural gas to generate high temperature and pressure steam. Steam is injected into the center, instead of the periphery of the rotor. This central injection requires a special coupler for flowing steam into the turbine.
There are other gas turbines or steam engines that are more efficient than impact or reaction turbines, such as the Watts steam engine using positive displacement of a piston, or the industrial gas turbine using lift of turbine blades that resemble air-foils. These engines are typically of a large size, noisy, comprise a large number of parts, and may rotate at high speed. This disclosure, through cutting spirals in solid disks of metal, creates a turbine that has very few parts, costs less to manufacture, and is failsafe. This disclosure therefore achieves the purpose of creating small and simple turbines which are as efficient as the large gas turbines. The turbine is suitable for small solar or thermal energy sources.
We give a more detailed historical development of turbines and steam engines in the remainder of this background survey.
Turbines are rotary machines that absorb energy from or impart energy to a moving fluid. Turbines are the subject of invention since antiquity. For example Hero invented a rotary boiler that ejected steam from two nozzles in opposite directions at the two ends of a diameter. The boiler spins in reaction to the steam ejected. The Hero turbine works by the principle of mechanical reaction.
Archimedes invented the famous Archimedes screw that uses rotary motion of a cork screw inside a cylinder to lift water. The Archimedes screw works by the principle of mechanical impact, imparting lift to water by the turbine blade. Many hydro-electric stations nowadays use the same impact or impulse principle for the reversed process of moving the turbine. Falling water makes impact on the turbine, turning the turbine which in turn drives an electrical generator.
Numerous turbines have been invented since the industrial revolution to convert heat, pressure, or motion power of a gas to perform industrial work or generate electricity. Here we cite a few notable inventions based on distinctive motive principles. First we note the Navier-Stokes equation
      ρ    ⁢                  D        ⁢                                  ⁢        v                    D        ⁢                                  ⁢        t              =            -              ∇        p              +          ∇              ·        T              +    f  which relates the density of the gas ρ, gas velocity vector v, gas pressure p, stress tensor T on the gas, and body force vector f on the gas through differential operators. The Navier-Stokes equation is the gaseous analog of Newton's second law of motion Ma=F, which states that the force F acting on a mass M would produce acceleration a=F/M. The material derivative
  ρ  ⁢            D      ⁢                          ⁢      v              D      ⁢                          ⁢      t      on the left of the Navier-Stokes equation is analogous to the term Ma in Newton's second law, whereas the three terms on the right of the equation represent the internal and external forces acting on the gas causing kinetic variation of the gas. The first term −∇p relates to the spatial gradient of change in the gas pressure. In our disclosure here, we want the gradient to be small as pressure drops continuously and slowly along the length of the spiral. The second term ∇·T relates to the stress force tensor T on the gas such as that caused by viscosity. This term is significant in the Tesla turbine but insignificant within our spiral turbine as the gas flow is not laminar. The last term f is the reaction of the turbine on the gas. The action of the gas on the turbine causes the turbine to spin, and in the process the heat and pressure energy of the gas is changed into kinetic energy.
Reaction turbines work when superheated and high pressure gas is forced through a De Laval nozzle. Most of the heat and pressure energy of a gas is converted within the short nozzle neck into a high speed and often supersonic jet of low pressure and temperature. In other words, −∇p changes rapidly within the nozzle only. The ejected gas provokes an equal and opposite reaction of the nozzle. Unfortunately, reaction turbine often spins at a dangerous speed of more than 10,000 revolutions per minute (rpm) while providing very little torque.
Impact or impulse turbines have stationary nozzles, for which high speed gas from a nozzle is forced to impact rotary turbine blades. Impact turbines also tend to turn at a very high speed with little torque. Impact turbines often are noisy when high speed gas hits the blades. Worse, impact turbines often have low efficiency as a high speed jet rapidly loses kinetic energy in turbulent impact with ambient air or turbine blades.
Most of the world's electricity is generated by steam turbine working on the principles of the Rankine engine. The modern steam turbine has alternating stages of rotating blades and static flow directors. High pressure and temperature steam enters a rotary turbine. Both the steam turbine and the wind turbine operate by the lift principle similar to that of an airplane wing. As gas accelerates on the upper surface of a wing, the lower surface experiences a lift due to higher pressure exerted by the slower flowing gas. Thus the turbine blade is forced to rotate by the aerodynamic lift. The flow exiting the turbine blades are redirected by a static channel. The redirected gas flow then hits at a correct angle on a second stage of turbine blades. The process of redirection and lift repeats for subsequent stages of turbine blades.
The high efficiency of the lift turbine is due to a non-turbulent flow of gas lifting the turbine blades, without a direct entropic impact on the turbine blades. The gas flows slowly pass the blade, expanding slowly while yielding a small part of its pressure to lift the blade. The slightly depressurized gas with lowered temperature can perform further work on the next stage of turbine blades. For modern combination cycle gas turbines, more than 60% of the heat energy from burning natural gas can be converted into mechanical work or electrical power.
Modern steam or gas turbines are powerful, large, and efficient. Unfortunately, they are complex, comprising of many moving parts rotating at high speeds. These turbines are therefore expensive to make and difficult to maintain.
The burning of fossil fuels for large centralized power generation generates billions of tons of carbon dioxide each year, causing global warming and depletion of fuel resources. Distributed renewable power generation, such as that provided by small solar thermal collectors or household heat sources, requires turbines that are small, simple, efficient, cheap, and reliable. Such turbines are yet to be made available on a large scale.
Our disclosure achieves these goals, using a distinctly novel spiral turbine based on the pressure and temperature principle. There are engines that perform work based on the pressure principle, such as the positive displacement of pistons for the classical steam engine of James Watt. Rotary steam engines use rotary vanes for rotation, unlike the Watts steam engines that use a crankshaft and flywheel to turn the linear motion of a piston into rotary motion.
These positive displacement engines are not as efficient as the modern gas and steam turbines, as gas flow is not smooth. Valves, crankshafts, seals, and flywheels operate sporadically. They are difficult to build and maintain at high temperature, pressure, and frequency of motion.
Our disclosure is distinctly different from these positive displacement engines, which operate by injecting a fixed volume of high pressure gas into a closed chamber and then allowing the gas to expand and push a piston or vane. Our gas flow is continuous and open, with no valves, piston, or closed housings. Due to the continuous and smooth gas flow, our invention has the advantage of smooth power delivery and balanced motion.
The Tesla turbine invented a century ago is re-emerging as a small form factor turbine for renewable power generation. The Tesla turbine works by aerodynamic drag resulting from the viscosity of a flowing gas. Gas is injected into the periphery of a stack of circular disks. The gas flows continuously between the disks, spiraling towards the center of the stack which serves as the gas exhaust. Drag force ∇·T corresponds to the second term on the right hand side of the Navier-Stokes equation
      ρ    ⁢                  D        ⁢                                  ⁢        v                    D        ⁢                                  ⁢        t              =            -              ∇        p              +          ∇              ·        T              +          f      .      The viscosity of the gas drags the disks, causing the stack to rotate in the same direction of the gas rotation. Gas flow is laminar between adjacent disks, with higher velocity midway between disks than at the surface of the disks. This laminar flow creates viscous drag on the disks as described by the Navier-Stokes equation.
Nicolas Tesla faced initially the problem of high spin velocity exceeding 10,000 rpm for his Tesla turbine. The high spin velocity of gas flow, coupled with less advanced machining and material technologies then, made the Tesla turbine less efficient and difficult to make. Since then, gas and steam turbine with rotary blades have become the dominant engine used for industrial power generation.
Our disclosure is distinctly different from the Tesla turbine. Besides using the pressure principle instead of viscous drag, the mechanical structure is different. For our turbine, gas flows from the disks center to the peripheral of the same disk in guided spirals, while the Tesla turbine flows between disks from peripheral to the center. For our turbine, gas flows in reverse direction of the turbine spin, as spin is caused by a reaction of the turbine to gas pressure.
For the Tesla turbine, gas drags the turbine along in the same direction. The viscous drag is acting on the disk surfaces, which are perpendicular to the spin axle of the Tesla turbine. For our turbine, the gas presses against the width of the flowing channel circumferential to the spin axle. The Tesla turbine requires housing for the spinning stack of disks to contain the gas on the circumference, while the gas spirals towards the center to exit. Our turbine requires gas to enter through a spinning axle to work against a spinning spiral on its way out at the periphery of the disks. Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages. Additionally, other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description.